Make the sauce: In a small bowl, whisk together soy sauce, chili crisp, lemon juice, sesame oil, and grated ginger until combined. Set aside so the flavors meld.
Prepare the tuna: Pat the tuna dry with paper towels. Coat in toasted sesame seeds.
Heat a heavy skillet (cast iron works best) over high heat. Add the neutral oil. Once shimmering hot, sear the tuna for about 45 seconds per side. The goal is a crisp, smoky exterior while keeping the interior nearly raw. Immediately transfer to a cutting board. Let rest for 1–2 minutes.
Slice: With a very sharp knife, slice the tuna crosswise into thin pieces (about ¼-inch thick).
Assemble: Arrange the slices on a chilled plate in a fan or overlapping row. Spoon some of the sauce over the tuna, and serve the rest on the side for dipping. Garnish with scallions, sesame seeds, and/or radish, if using.
